Definitions
Electromagnetic Radiation to Electric Energy Conversion Device (EREECD): A device that reacts with electromagnetic (optical) radiation to produce electrical energy
Optical Radiation to Electric Energy Conversion Device (OREECD): A device that reacts with optical electromagnetic radiation to produce electrical energy. Such a device could be a radiation absorbing device, e.g. a photodetector/counter, photovoltaic cell (solar cell) or a radiation-driven electrolysis cell.
Optoelectronic Energy Device (OED): A device that reacts with optical radiation to produce electrical energy with an electronic device.
Photovoltaic cell: An electrical device (e.g. a semiconductor) that converts light or other radiant energy, in the range from ultraviolet to infrared radiation, incident on its surface into electrical energy in the form of power/voltage/current which has two electrodes, usually a diode with a top electrode and a bottom electrode with opposite electrical polarities. The photovoltaic cell produces direct current which flows through the electrodes. As employed herein, the term photovoltaic cell is generic to cells which convert radiant energy into electrical energy including EREECDs, OREECDs, and OEDs as defined above.
Solar cell: An electrical photovoltaic device (e.g. a semiconductor) that converts light incident on its surface into electrical energy which has two electrodes, usually a diode with a top electrode and a bottom electrode with opposite electrical polarities. The solar cell produces direct current which flows through the electrodes. As employed herein, the term solar cell is generic to cells which convert radiant energy into electrical energy.
This invention relates to tandem photoelectric devices formed from separate photovoltaic cells and more particularly to interconnections therebetween.
Multijunction photovoltaic cells are devices containing two or more photovoltaic cells of different bandgaps connected in series and exposed to sunlight such that higher energy photons are absorbed in the upper-lying photovoltaic cells with their higher bandgaps while lower energy photons are transmitted to the lower bandgap photovoltaic cells in the stack. This combination makes more efficient use of the solar spectrum by converting more of it to electricity rather than heat. For example, single junction photovoltaic cells have a maximum efficiency of approximately 30% while multijunctions can exceed 50%, and 40% efficient triple junction photovoltaic cells have already been demonstrated. Such photovoltaic cells can be made in part by epitaxial growth of all the materials and layers.
For example, a triple junction photovoltaic cell, consists of a Ge N+/P junction, a P+/P/N+ GaAs photovoltaic cell grown upon the Ge, a P+/P/N+ GaInP junction grown upon the GaAs, with the P+/N+ boundaries acting as tunnel junctions, a AlInP passivating window layer upon the GaInP, a GaAlAs passivating window layer grown upon the GaAs, totaling twelve to seventeen layers of semiconductor material each of which must be lattice matched to all the others to prevent lattice mismatch defect formation, and each has to have an accurately determined thickness to ensure equal photocurrents in each photovoltaic cell (currents must be equal in each photovoltaic cell of an electrically series connection to obtain maximum power output and prevent anyone of the cells acting as a power-draining efficiency-lowering load upon the others). A major problem can arise due to the need to prevent diffusion of dopant from any of the layers into adjacent regions. Such cross diffusion raises the resistance of the tunnel junctions and lowers efficiency. This sets a limit on the temperature/time profile during fabrication. In semiconductor fabrication, the simultaneous requirements of tight control of many variables (temperature, time, thicknesses, lattice matching, dopant densities) generally lowers yield and raises costs.
Wafer bonding represents an improved process for creating multijunction photovoltaic cells. By creating photovoltaic cells of different materials and bandgaps separately and joining them by bonding, each photovoltaic cell can be optimized and yield can be raised. Such bonded tandem photovoltaic cells are described, e.g. Zahler et al U.S. patent publication 2006/0021565 that discusses creating a GaAs/GaInP dual junction tandem photovoltaic cell by monolithic epitaxy means and bonding this to a silicon photovoltaic cell/substrate. Exfoliation (separation of the desired device volume from the handle substrate) is generally induced by low ion mass ion implantation which causes a weak zone in the material which cracks off upon heating if the ion density is sufficient. Bonding by electrostatic means (applying a high electric field) is described in Stambery U.S. Pat. 5,261,969.
One means of increasing the efficiency of photovoltaic cells is to incorporate quantum well layers within the depletion region of the photovoltaic cell p/n junction. Such quantum wells may be alternating layers of materials having a thickness less than ten nanometers with lower bandgaps than the host junction in order to increase the sunlight wavelength range over which the photovoltaic cell operates. Such quantum well incorporation has been discussed in Moustakas WO2005/104236, Moustakas et al. U.S. patent publication 2005/0242364, and Suzuki U.S. patent publication 2002/0050288A1 which also describe texturing the surface of the upper photovoltaic cell to reduce light reflection.
Quantum well incorporation and tunnel junction formation in monolithic tandem photovoltaic cells are discussed in Freundlich U.S. Pat. No. 6,372,980. The above mentioned W020051104236, Park et al U.S. Pat. 6,663,944, Ji et al. U.S. Pat. No. 6,420,647, and Shaharyar U.S. Pat. No. 5,258,077 describe the use of textured surfaces to result in light trapping in thin films of semiconductors, such that light which is not absorbed in the first pass through the material will make multiple passes which increases its absorption probability. Fabricating tandem photovoltaic cells with a metal interconnect between intermediate cells in place of a tunnel junction is described in Manlass U.S. Pat. No. 5,322,572.